Abstract
Ionic mechanisms of excitation were studied in the immature egg cell membrane of a starfish, Mediaster aequalis, by analyzing membrane currents during voltage clamp. The cell membrane shows two different inward current mechanisms. One is activated at a membrane potential of - 55 approximately -50 mV and the other at -7 approximately -6 mV. They are referred to as channels I and II, respectively. A similar difference is also found in the membrane potential of half inactivation. Currents of the two channels can, therefore, be separated by selective inactivation. The currents of both channels depend on Ca++ (Sr++ or Ba++) but only the current of channel I depends on Na+. The time-course of current differs significantly between the two channels when compared at the same membrane potential. The relationship between the membrane current and the concentration of the permeant ions is also different between the two channels. The result suggests that channel II is a more saturable system. The sensitivity of the current to blocking cations such as Co++ or Mg++ is substantially greater in channel II than in channel I. Currents of both channels depend on the external pH with an apparent pK of 5.6. They are insensitive to 3 muM tetrodotoxin (TTX) but are eliminated totally by 7.3 mM procaine. The properties of channel II are similar to those of the Ca channel found in various adult tissues. The properties of channel I differ, however, from those of either the typical Ca or Na channels. Although the current of the channel depends on the external Na the amplitude of the Na current decreases not only with the Na concentration but also with the Ca concentration. No selectivity is found among Li+, Na+, Rb+, and Cs+. The experimental result suggests that Na+ does not carry current but modifies the current carried by Ca in channel I.
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Selected References
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- Baker P. F., Hodgkin A. L., Ridgway E. B. Depolarization and calcium entry in squid giant axons. J Physiol. 1971 Nov;218(3):709–755. doi: 10.1113/jphysiol.1971.sp009641. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Chandler W. K., Meves H. Voltage clamp experiments on internally perfused giant axons. J Physiol. 1965 Oct;180(4):788–820. doi: 10.1113/jphysiol.1965.sp007732. [DOI] [PMC free article] [PubMed] [Google Scholar]
- FATT P., GINSBORG B. L. The ionic requirements for the production of action potentials in crustacean muscle fibres. J Physiol. 1958 Aug 6;142(3):516–543. doi: 10.1113/jphysiol.1958.sp006034. [DOI] [PMC free article] [PubMed] [Google Scholar]
- FATT P., KATZ B. The electrical properties of crustacean muscle fibres. J Physiol. 1953 Apr 28;120(1-2):171–204. doi: 10.1113/jphysiol.1953.sp004884. [DOI] [PMC free article] [PubMed] [Google Scholar]
- HAGIWARA S., NAKA K. I. THE INITIATION OF SPIKE POTENTIAL IN BARNACLE MUSCLE FIBERS UNDER LOW INTRACELLULAR CA++. J Gen Physiol. 1964 Sep;48:141–162. doi: 10.1085/jgp.48.1.141. [DOI] [PMC free article] [PubMed] [Google Scholar]
- HODGKIN A. L., HUXLEY A. F. Currents carried by sodium and potassium ions through the membrane of the giant axon of Loligo. J Physiol. 1952 Apr;116(4):449–472. doi: 10.1113/jphysiol.1952.sp004717. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Hagiwara S., Nakajima S. Differences in Na and Ca spikes as examined by application of tetrodotoxin, procaine, and manganese ions. J Gen Physiol. 1966 Mar;49(4):793–806. doi: 10.1085/jgp.49.4.793. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Kidokoro Y. Development of action potentials in a clonal rat skeletal muscle cell line. Nat New Biol. 1973 Jan 31;241(109):158–159. doi: 10.1038/newbio241158a0. [DOI] [PubMed] [Google Scholar]
- Meves H., Vogel W. Calcium inward currents in internally perfused giant axons. J Physiol. 1973 Nov;235(1):225–265. doi: 10.1113/jphysiol.1973.sp010386. [DOI] [PMC free article] [PubMed] [Google Scholar]
- Miyazaki S., Takahashi K., Tsuda K. Calcium and sodium contributions to regenerative responses in the embryonic excitable cell membrane. Science. 1972 Jun 30;176(4042):1441–1443. doi: 10.1126/science.176.4042.1441. [DOI] [PubMed] [Google Scholar]
- NARAHASHI T., MOORE J. W., SCOTT W. R. TETRODOTOXIN BLOCKAGE OF SODIUM CONDUCTANCE INCREASE IN LOBSTER GIANT AXONS. J Gen Physiol. 1964 May;47:965–974. doi: 10.1085/jgp.47.5.965. [DOI] [PMC free article] [PubMed] [Google Scholar]
- TAYLOR R. E. Effect of procaine on electrical properties of squid axon membrane. Am J Physiol. 1959 May;196(5):1071–1078. doi: 10.1152/ajplegacy.1959.196.5.1071. [DOI] [PubMed] [Google Scholar]